1. Field of the Invention
The present invention relates generally to wireless communication systems and, more particularly, to a structure of scheduling assignments for the transmission of data signals.
2. Description of the Art
A communication system consists of a DownLink (DL), supporting the transmission of signals from a base station (Node B) to User Equipments (UEs), and an UpLink (UL), supporting the transmission of signals from UEs to the Node B. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A Node B is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
DL signals consist of data signals, carrying information content, control signals, and Reference Signals (RS), which are also known as pilot signals. The Node B conveys DL data signals through a Physical Downlink Shared CHannel (PDSCH). The UEs convey UL data signals through a Physical Uplink Shared CHannel (PUSCH). The DL control signals may be of a broadcast or a UE-specific nature. Broadcast control signals convey system information to all UEs. UE-specific control signals can be used, among other purposes, to provide, to UEs, Scheduling Assignments (SAs) for PDSCH reception (DL SAs) or PUSCH transmission (UL SAs). SAs are transmitted through a Physical Downlink Control CHannel (PDCCH).
The PDCCH is usually a major part of the total DL overhead and directly impacts the achievable DL system throughput. One method for reducing the PDCCH overhead is to scale the PDCCH size according to its required resources during each Transmission Time Interval (TTI). In 3GPP Long Term Evolution (LTE), where the Node B uses Orthogonal Frequency Division Multiple Access (OFDMA) as the DL transmission method, a Control Channel Format Indicator (CCFI) parameter transmitted through a Physical Control Format Indicator CHannel (PCFICH) indicates the number of Orthogonal Frequency Division Multiplexing (OFDM) symbols occupied by the PDCCH.
A structure for the PDCCH and PDSCH transmission in the DL TTI is shown in FIG. 1. The DL TTI is assumed to consist of a single sub-frame 110 having M OFDM symbols. A PDCCH 120 occupies the first N OFDM symbols and a PDSCH 130 occupies the remaining M-N OFDM symbols. A PCFICH 140 is transmitted in some sub-carriers, also referred to as Resource Elements (REs), of the first OFDM symbol. Some OFDM symbols may contain RS REs, 150 and 160, for each of the Node B transmitter antennas. In FIG. 1, it is assumed that there are two Node B transmitter antennas. Among the main purposes of the RS are to enable a UE to obtain an estimate for the DL channel medium it experiences and to perform other measurements and functions as they are known in the art. Additional control channels may be transmitted in the PDCCH region but, for brevity, they are not shown in FIG. 1. For example, assuming the use of Hybrid Automatic Repeat reQuest (HARQ) for PUSCH transmissions, a Physical Hybrid-HARQ Indicator CHannel (PHICH) may be transmitted by the Node B to indicate to UEs whether their previous PUSCH transmissions were correctly or incorrectly received by the Node B.
The Node B separately encodes and transmits each of the UL SAs and DL SAs in the PDCCH. An SA encoding process is illustrated in FIG. 2. The DL SA or UL SA information bits 210, respectively conveying the information scheduling PDSCH reception or PUSCH transmission by a UE, are appended with Cyclic Redundancy Check (CRC) bits in step 220, and are subsequently encoded in step 230, for example using a convolutional code. The bits are rate matched to the assigned PDCCH resources in step 240, and transmitted in step 250. As a consequence, each UE may perform multiple decoding operations to determine whether it is assigned a DL SA or an UL SA in the corresponding sub-frame. Typically, the CRC of each SA is scrambled with an IDentity (ID) of the UE the SA is intended for. After descrambling using its ID, a UE can determine whether an SA is intended for the UE by performing a CRC check.
At the UE receiver, the inverse operations are performed to decode an SA as illustrated in FIG. 3. The received SA 310, is rate de-matched in step 320, decoded in step 330, and after the CRC is extracted in step 340, the SA information bits are obtained in step 350. As previously described, if the CRC check passes, the SA is considered to be intended for the UE.
A structure for the PUSCH transmission in the UL TTI, which is assumed to consist of one sub-frame, is shown in FIG. 4. Single-Carrier Frequency Division Multiple Access (SC-FDMA) is assumed to be the transmission method. A sub-frame 410 includes two slots. Each slot 420 includes seven symbols used for the transmission of data or control signals. Each symbol 430 further includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. PUSCH transmission in one slot may be in the same or different part of the operating BandWidth (BW) than the PUSCH transmission in the other slot. PUSCH transmission in different BWs in each slot is referred to as Frequency Hopping (FH). Some symbols in each slot may be used for RS transmission 440 to provide channel estimation and to enable coherent demodulation of the received signal. The transmission BW is assumed to consist of frequency resource units, which are referred to as Physical Resource Blocks (PRBs). Each PRB is further assumed to consist of NscRB REs, and a UE is allocated MPUSCH consecutive PRBs 450 for its PUSCH transmission.
A conventional UL SA is described through an set of Information Elements (IEs) in Table 1. Additional IEs or a different number of bits for the indicative IEs in Table 1 may apply. The order of the IEs in a UL SA can be arbitrary. The length of the CRC (UE ID) is assumed to be 16 bits but other values, such as 20 bits or 24 bits, may be used instead.
TABLE 1IEs of an UL SA for PUSCH Transmission in Contiguous PRBsNumber ofInformation ElementBitsCommentIndication of UL SA1Indicates that the SA is for ULTransmissionResource Allocation (RA)11Assignment of Consecutive PRBs (total 50 PRBs)Modulation and Coding 5MCS LevelsScheme (MCS)New Data Indicator (NDI)1New Data Indicator (synchronous HARQ)Transmission Power Control2Power control commands(TPC)Cyclic Shift Indicator (CSI)3SDMA (maximum of 8 UEs)Frequency Hopping (FH)1Frequency Hopping (Yes/No)Channel Quality 1Include CQI report (Yes/No)Indicator (CQI)RequestUnused Bit1To align the UL SA size with a DL SA sizeCRC (UE ID)16UE ID masked in the CRCTOTAL42
The first IE differentiates the UL SA from an SA used for a different purpose, such as, for example, for PDSCH scheduling (DL SA). The UL SA and the DL SA are desired to have the same size in order for both SAs to be examined with a single decoding operation at the UE.
The second IE is a Resource Allocation (RA) IE, which specifies the assigned PRBs for PUSCH transmission. With SC-FDMA, the signal transmission BW is contiguous. For an operating BW of NRBUL PRBs, the number of possible contiguous PRB allocations to a UE is 1+2+ . . . +NRBUL=NRBUL(NRBUL+1)/2 and can be signaled with (┌log2(NRBUL(NRBUL+1)/2)┐) bits, where ┌ ┐ denotes the “ceiling” operation which rounds a number towards its next higher integer. Therefore, for an operating BW of NRBUL=50 PRBs assumed in Table 1, the number of required bits is 11. In general, regardless of the transmission method, the UL SA is assumed to contain an RA IE.
The third IE indicates a Modulation and Coding Scheme (MCS) for the PUSCH transmission. With 5 bits, a total of 32 MCS values can be supported. For example, the modulation may be QPSK, QAM16, or QAM64, while the coding rate may take discrete values between, for example, 1/16 and 1. Some values of the MCS IE may be reserved to be used in support of HARQ. For example, the last 3 of the 32 MCS values may be used to indicate a Redundancy Version (RV) for a packet retransmission for the same Transport Block (TB). In that case, the MCS is determined from the MCS of the previous SA for the same TB, which is assumed to be specified with one of the first 29 MCS values.
The fourth IE is a New Data Indicator (NDI). The NDI is set to 1 if a new TB should be transmitted, while it is set to 0 if the same TB, as in a previous transmission, should be transmitted by the UE (synchronous HARQ is assumed).
The fifth IE provides a Transmission Power Control (TPC) command for power adjustments of the PUSCH transmission. For example, the 2 bits of the TPC IE in the UL SA, [00, 01, 10, 11], may respectively correspond to [−1, 0, 1, 3] deciBel (dB) adjustments of the PUSCH transmission power.
The sixth IE is a Cyclic Shift (CS) Indicator (CSI) enabling the use of a different CS for a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence assumed to be used for RS transmission in FIG. 4. The different CS of a CAZAC sequence, adequately separated in time, can result in orthogonal CAZAC sequences. This property can be used to orthogonally multiplex the RS transmission from different UEs in the same PRBs, in order to support Spatial Division Multiple Access (SDMA) for PUSCH transmissions.
The seventh IE indicates whether the UE should apply FH to its PUSCH transmission. For example, if the FH IE value is set to 1, the UE applies FH to its PUSCH transmission as previously explained and described in greater detail below.
The eighth IE indicates whether the UE should include a Channel Quality Indicator (CQI) report in its PUSCH transmission. The CQI report provides the Node B with information about channel conditions the UE experiences in the DL. This information can enable the Node B to select parameters for PDSCH transmission to that UE, such as the MCS and PRBs, so that a performance metric, such as the system throughput or the UE throughput, is improved.
The ninth IE is an unused bit, set to a predetermined value such as 0, which is assumed to be needed to pad the UL SA size in order to make it equal to the size of a DL SA.
The transmission mode for the UL SA described in Table 1 corresponds to PUSCH transmission from a single UE antenna or to antenna transmission diversity. A different UL SA can be defined for a transmission mode corresponding to PUSCH transmission from a UE using a Multiple Input Multiple Output (MIMO) transmission principle.
In an FH operation, a total number of PUSCH PRBs is defined as NRBPUScH=NRBUL−NRBHO and the parameter “PUSCH-HoppingOffset” is defined as NRBHO, which is provided to the UEs by higher layers. The PUSCH transmission in the first slot is at the PRBs specified by the RA IE in the UL SA, and the PUSCH transmission in the second slot is at an equal number of PRBs whose starting point is obtained by adding └NRBPUSCH/2┘ to the starting point of the PRBs in the first slot, where └ ┘ is the “floor” operation which rounds a number to its immediately lower integer. The FH operation is illustrated in FIG. 5 where NRBUL=50 PRBs 510, NRBHO=10 PRBs 520, which are equally divided on each side of the BW, and NRBPUSCH=40 PRBs 530. A total of 5 PRBs 540 are allocated to the PUSCH transmission by a UE starting from PRB 11 550 in the first slot and PRB number 31 560 in the second slot. Several other realizations of the FH operation are also possible.
In addition to SC-FDMA, where the signal transmission is over a contiguous BW (single cluster of consecutive PRBs with RA IE as described in Table 1), the same transmitter and receiver structure can be used for signal transmission over multiple clusters (non-contiguous sets of PRBs). Because a Discrete Fourier Transform (DFT) is applied to the signal transmission, this method is known as DFT-Spread-OFDM (DFT-S-OFDM). For a single cluster, DFT-S-OFDM is identical to SC-FDMA. For a number of clusters equal to the number of REs in the operating BW, DFT-S-OFDM becomes identical to conventional OFDM.
A block diagram of the transmitter functions for clustered OFDM signaling is illustrated in FIG. 6. Encoded data bits 610 are applied to a DFT 620, RE mapping 630 for the assigned transmission BW are selected through control of localized Frequency Division Multiple Access (FDMA) 640 (zeros are mapped to non-selected REs). Inverse Fast Fourier Transform (IFFT) 650 and CP insertion is performed, time windowing filtering 670 is applied and the signal 680 is transmitted. Additional transmitter circuitry such as a digital-to-analog converter, analog filters, and transmitter antennas are not shown. Also, the encoding and modulation process for the data bits is omitted. The selected REs after the DFT may be in a single cluster of contiguous REs 690 or they may be in multiple clusters of contiguous REs 695.
At the receiver, the reverse (complementary) transmitter operations are performed as illustrated in FIG. 7. After an antenna receives a Radio-Frequency (RF) analog signal and after further processing units (such as filters, amplifiers, frequency down-converters, and analog-to-digital converters) which are not shown, digital signal 710 is filtered at time windowing 720 and continues through CP removal 730. Subsequently, the receiver unit applies an FFT 740, demaps the REs 760 used by the transmitter through control of the reception bandwidth 750 (zeros are appended for the remaining REs), applies an Inverse DFT (IDFT) 770 and obtains received coded data bits 780. Well known receiver functionalities such as channel estimation, demodulation, and decoding are not shown.
There are several issues associated with the design of the control signaling required for supporting contiguous PRB allocations in conjunction with the control signaling required for supporting non-contiguous PRB allocations for a given transmission mode.
A first issue is to avoid introducing different UL SA sizes depending on the number of clusters specified by the RA IE in the UL SA. Assuming that the remaining IEs, as described in Table 1, remain unchanged, different RA IE sizes for addressing a different number of PRB clusters will lead to different UL SA sizes. Since a UE cannot know in advance the number of its allocated PRB clusters, it will have to decode each UL SA corresponding to each possible RA size. This will lead to an increase in the number of decoding operations the UE needs to perform and a respective increase in the PDCCH decoding complexity. For example, if allocations of one cluster of PRBs and allocations of two clusters of PRBs are supported, with each requiring a different UL SA size, the number of decoding operations for the UL SAs is doubled relative to their respective number when only allocation of one cluster of PRBs is supported.
A second issue is that by allowing a large number for clusters of PRBs to be allocated, the respective size of the RA IE in the UL SA may substantially increase, thereby leading to an increase in the total UL SA size and an increase in the associated PDCCH overhead.
Therefore, there is a need to support control signaling for scheduling PUSCH transmissions over non-contiguous PRB allocations by limiting the number of PRB clusters addressable in the RA IE of the respective UL SA.
There is another need to avoid increasing the number of decoding operations associated with UL SAs supporting PUSCH transmissions over non-contiguous PRB allocations.
Finally, there is another need to maintain a small UL SA size for supporting PUSCH transmissions over non-contiguous PRB allocations to avoid increasing the PDCCH overhead.